Thing is if you're in flowering then you're probably giving them a nute that has some potassium in it. This should be enough to fix an iron def. problem.

I believe your problem is ph based.

Take one of your pots. Drill 1 to 2 holes per side (use 4 sides North, east, south, west) into your pots. Take one of these pots and keep pouring water through it (without nutes but ph'd at what you normally use your water ph'd at) until you start to get run off.

Capture this run off. For example I capture mine in a measuring cup and use a pen PH meter once there is about 2-4oz in the cup.

I make sure to stir it up also to make sure to get an accurate reading.

I believe for a soiless medium you want to be between 5.8 and 6.2.

Again I'm no expert. May want to wait for a pro to come in before doing anything drastic.